Filling the Gaps between the In Vivo and In Vitro Microenvironment: Engineering of Spheroids for Stem Cell Technology.

Engineering of in vitro three-dimensional cultures of stem cells and their progenies has offered promising alternatives to recapitulate the in vivo microenvironment, or stem cell niche, and has provided more specific cues for proper stem cell differentiation, maintenance and culture. In particular, tissue spheroids are cellular aggregates with defined cellular and extracellular features and have provided optimal conditions for stem cell technology, both in culture and for potential engraftment. Recent studies have focused on spheroid formation and the developmental roles played by cellular and extracellular signals necessary for cellular aggegation into spheroids. This review will provide insights into the factors that regulate in vitro spheroid formation by comparing them with their developmental counterparts in vivo. At the same time, we will identify cellular and extracellular signals that could be used to bioengineer spheroids with improved features according to their application. Finally, this review will provide an overview of the applications to date of spheroid cultures of stem cells and their progenies, providing insights for future studies.